Production of recombinant proteins in cell culture normally involves a series of purification steps, by which the desired protein product is recovered from recombinant host cells and/or the associated culture media. Many important recombinant proteins are produced on a large commercial scale. In the case of pharmaceutical proteins, for example, it is not uncommon for more than one purification stage to be used to achieve the desired level of product purity.
It can be necessary to store a bulk solution of recombinant protein which has been initially purified, but not finally purified, prior to final purification for formulation. For example, a protein-containing product of a recombinant fermentation reaction can be initially purified in an affinity or ion exchange column. After an initial pass through the column, the protein product is only partially purified, and the solution still contains contaminants such as remnants of the cell culture and other proteins. Prior to final formulation into a pharmaceutical product, the bulk solution must be further processed to obtain the protein in a satisfactory purity.
Normally the solution, e.g. elution buffer, which is used to recover the protein from a first-pass purification treatment is a high salt solution. In the case of elution from a column, a high salt concentration is needed to release the protein from the column. Accordingly, the “bulk” solution recovered from first pass purification treatment can comprise a solution having a high concentration of monovalent salts, normally sodium chloride but potentially potassium chloride, or other salts.
The storage of a “bulk” solution of recombinant protein poses unique challenges due to the high salt concentration and very low protein concentration of the solution. Ideally, proteins are stored below the glass transition temperature to assure stability, since in the glassy state, protein inactivation and denaturation are extremely slow on a pharmaceutical time scale. On the other hand, the presence of high salt concentration in a solution tends to depress its glass transition temperature, and in solutions with high salt concentration and low protein concentration, very low temperatures are needed to achieve this state.
Frozen storage at higher temperature is desirable for bulk solutions in large volume quantities for cost and efficiency reasons, but while preserving the stability and activity of the protein.